Add mail notification and event journal

Update README.md

Fix typo

The mail notification is enabled if a mailSender is present specified via spring.mail.* properties.
On default the mail is sent to root@localhost and the chages from UNKNOWN to UP are ignored.

Added a simple event journal, jorunaling all ClientApplicationEvents using a simple, non-persistent store. So all information is lost after restart. Added corresponding view to UI.

The health-check is made at a fixed interval (default: 10s) for all expired status-information (default-lifetime: 30s).
The application health-endpoint must either respond with a JSON-Body having a status-field or with an empty body and all 2xx-http-responses are interpreted as UP and every other status as DOWN.

Fix typo.

If `spring.boot.admin.client.useIpAdressOf=<network-interface>` is set properly the ip-address of the specified network-interface is used in the inferenced url to register with.
In case the address cannot be determined an exception is thrown and the client won't register.
This is for example useful in Docker environments making the inferenced urls work.

closes #74

closes #69

This allows to (re-)set properties via POST-requests to the environment endpoint.

fixes #71

Checks for the presence of Ssl in ServerProperties and that it is
enabled, uses https in that case for client URL. Added test case for
option.

If an application is changing state while the page is open a desktop notification is shown.

Activiti spring boot starter exposes the /activiti endpoint which is now exposed in the admin ui. See https://spring.io/blog/2015/03/08/getting-started-with-activiti-and-spring-boot for further details on creating a client

Fix typo.

fixup! Add optional automatic deregistration on shutdown - turned off by default; only periodically try to register when the context is active

Add optional automatic deregistration on shutdown - turned off by default; only periodically try to register when the context is active